This invention relates to a method and apparatus for providing additional or replacement anodes to retrofit cathodic protection systems to control the corrosion of marine structures.
Present day offshore platforms used in the oil and gas industry are often formed of large-diameter pipe elements in the form of three or more vertical or slanting legs interconnected or reinforced by cross-bracing tubular members. Such bottom-supported platforms have been set in waters up to 1200 feet deep. The deepwater platforms may have more legs which may be tapered. In addition, the platform is provided with multiple vertical pipes, risers or well conductors which are grouped near the center of the platform and through which wells are drilled. Further, the platform supports vertical pipe risers through which oil and gas may be separately pumped down to an ocean floor pipeline and then via pipeline to shore or to another platform.
These offshore structures, pipelines and subsea installations are generally built from steel. Steel structures in seawater require cathodic protection to prevent rapid wastage by corrosion. For large, permanently installed structures, corrosion is usually prevented by the use of cathodic protection, sometimes supplemented with paints or coatings. In seawater, cathodic protection is generally applied by attaching either aluminum or zinc sacrificial anodes. The anodes corrode to produce an electrical current that protects the steel structure from corrosion. When these anodes are consumed by corrosion, they must be replaced, or retrofitted.
Corrosion in seawater is an electrochemical process. During the chemical reaction of metals with the environment to form corrosion products (such as rust on steel), metallic atoms give up one or more electrons to become positively charged ions, and oxygen and water combine to form negatively charged ions. The reactions occur at rates which result in no charge buildup. All the electrons given up by the metal atoms are consumed by the other reaction.
Cathodic protection is a process which prevents the corrosion reaction by creating an electric field so that current flows into the metal. This prevents the formation of metal ions by setting up a potential gradient at the surface, which opposes the electric current produced by flow of electrically charged ions away from the metal surface as the product of corrosion. The electric field must be of adequate strength to counter the field produced by the corrosion reaction to ensure that metal ions are fully prevented from escaping.
A source of the electric field which opposes the corrosion reaction may be a current supplied from the preferential corrosion of a metal anode with different electrochemical properties in the environment, and which has a stronger anodic reaction with the environment than does the offshore structure. Thus, current flows to the structure from the additional anode, which itself progressively corrodes in preference to the structure. This technique is known as sacrificial anode cathodic protection.
In order to protect the offshore platforms from corrosion in sea water, the structural members of the platform are typically provided with a cathodic protection system which is comprised of a multitude of sacrificial anodes, which are preferably made of alloys of aluminum or zinc, that are attached directly to the platform in a manner that has been in use since the 1960""s.
When a sacrificial anode system is chosen, the weight of material required to provide the protection current for the protected lifetime of the structure is calculated from a knowledge of the current demand of the structure and also the specific electrochemical properties of the anode alloy. The calculated weight of anode alloy cannot be installed all in one piece but must be distributed over the structure in the form of smaller anodes to ensure uniform distribution of current. In order to select the best size and shape of anode, the total current demand of the structure both at the beginning and end of its life must be considered. The anode must deliver adequate current to polarize the structure and build up cathodic carbonate scales on the metal surface, but also must be capable of delivering the required mean current for the structure when 90% consumed. Thus on most offshore platforms, a multiplicity of anodes are arranged on the various structural members of the platform. These anodes are generally attached to the platform before the platform is lowered into the ocean floor.
Cathodic protection of marine structures is usually designed to last for the life of the structure. When either the structure""s operational life is extended, or the original design requires supplementation prior to the end of life, a retrofit cathodic protection system must be installed. In shallow water, retrofit anodes are commonly installed by divers. For structures that are located in water that is deep enough to require saturation diving or remote operated vehicles (ROVs), the conventional practice can become prohibitively expensive.
For small sea bottom structures, such as the wellhead templates used to protect subsea production systems, a problem can arise with attaching sufficient anodes. The anodes needed to protect the template, the subsea equipment within the template, the well casings and the flowlines that carry fluids to and from the template must all be located on the template. The templates are desired to be small. There may not be a sufficient number of sites around the template that are suitable for the attachment of enough sacrificial anodes to provide the desired protection.
The placement of objects on the sea floor in the area around an offshore structure is strictly controlled. Safety concerns about anchors from ships and supply boats damaging the subsea pipelines usually require that all shipping activities be limited to one side of the platform and that the pipeline risers be located on the opposite side. For any objects that must be placed on the sea floor, one must consider the locations of pipelines as well as the potential damage that might be caused by anchors or propeller wash from ships or boats.
Offshore pipelines may also require retrofit of cathodic protection anodes. Most pipelines have anodes that are attached as the pipeline is being laid on the sea bottom. These anodes are designed to last for a specific number of years that corresponds with the original design life of the pipeline. In cases where the cathodic protection design was not correct or where the useful life of the pipeline extends beyond the original design life, it is necessary to retrofit anodes onto the pipeline. Since pipelines in shallow water are buried, this means that a diver must dig out a section of the pipeline, remove any external coatings from the pipe and then weld a new anode to the pipe. Since anodes are often installed every 500 to 1000 feet along the pipe, this can be extremely expensive. In the case of pipelines in deeper water, the pipeline may not be buried, but the pipeline will usually be at water depths where normal diving operations cannot be conducted. The use of saturation diving or remote operated vehicles (ROVs) to attach retrofit anodes to a pipeline is also very expensive.
Various types of anode retrofits have been used. Anode piles have been proposed by Shell (U.S. Pat. Nos. 4,484,839 and 4,484,840) to retrofit anodes on deepwater platforms. However, the use of anode piles requires pile guides, to physically hold the anode piles against the platform. Not all platforms have existing pile guides that are available for this use.
Anode sleds have also been used on both platforms and pipelines (see U.S. Pat. No. 4,609,307). The anodes are attached to sleds or pods that are placed on the ocean bottom and electrically connected to the structure or pipeline. However, fabrication and installation practices place practical limitations on the physical size of the sleds. This size restriction limits the number of anodes that can be attached and the spacing of anodes on the sled. When anodes are placed too close to each other on the sled, the anodes will start to electrically interfere with each other, thereby limiting the amount of electrical current that each sled can produce. On a large, deepwater structure, many sleds may be required to produce the desired amount of electrical current. The physical space around the structure available for such use may limit the number of sleds that can be installed, hence making this approach impractical.
It is an object of this invention to provide a novel, cost-effective way to retrofit cathodic protection systems on marine structures.
The subject invention is a cathodic protection system for offshore structures, which is particularly useful when used to retrofit offshore structures that have exceeded their expected life or require supplemental cathodic protection. Alternatively, the invention may also be applied to new offshore structures. The subject invention is particularly applicable to smaller structures, such as sub-sea templates or subsurface production systems, which may not have sufficient structural elements to provide attachment spots for a multiplicity of protecting anodes.
The anode system of the invention includes an elongated anode carrier with a plurality of sacrificial anodes attached, and a conductor cable to attach the electrode carrier to the protected offshore structure. The anode carrier is preferably a long pipe. Conventional anodes, available in various shapes and materials, are attached to the carrier. The anode carrier is generally very long, and may be longer than the depth of the ocean near the offshore structure.
The subject invention offers the advantage of what is, in effect, a multiplicity of anodes, that may be attached to an existing structure with as few as one connection. The anode system may be assembled, but for the final connection to the structure to be protected, above the water, and perhaps even in an onshore fabrication shop, depending upon the type of assembly utilized. The minimal number of connections to the protected structure minimizes the time that divers and/or ROVs must spend to make the under-water connections, thus simplifying installation and reducing costs. Placing the anode system on the sea bottom allows it to be supported by the ocean floor, which avoids the necessity of providing attachment points that will physically support the weight of each anode, in addition to providing electrical conductivity.